Project Summary/Abstract Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancer types with a devastating 7% five- year survival rate. At present, no effective treatment exists for PDAC patients. Gemcitabine, the first-line adjuvant chemo-therapy for advanced PDAC, only yields a mild therapeutic response in 6-11% of PDAC patients. Targeted- and immuno-therapies have also showed very limited efficacy in PDAC. EGFR is frequently activated in PDAC. Although an EGFR tyrosine kinase inhibitor (TKI), erlotinib, in combination with gemcitabine, has been approved by FDA for PDAC treatment, the combination only slightly improved PDAC patients? survival compared to gemcitabine alone (one year survival: 23% vs. 17%). It has been puzzling why EGFR/ErbB2 targeted therapies only had very limited benefits despite of frequent EGFR/ErbB2 activations in PDAC. Recently, we found that 14-3-3? overexpressing (14-3-3?+++) PDAC cells increase EGFR/ErbB2 activation and their growth were effectively inhibited by an EGFR/ErbB2 dual TKI (lapatinib) when cultured alone. However, the same 14-3-3?+++ and EGFR/ErbB2 activated PDAC cells were not inhibited by lapatinib when co-cultured with fibroblasts. The data phenotypically copy the low response to EGFR/ErbB2 TKIs in PDAC patients and suggested that the fibrotic tumor microenvironment (TME) may render PDAC tumor cells resistant to EGFR/ErbB2 TKIs. PDAC tumor cells and its fibrotic TME reciprocally communicate to co-evolve creating a vicious cycle. Indeed, we found that the 14-3-3?+++ PDAC cells secrete more CXCL5 which activates the PI3K-Akt-GSK3? signaling axis in fibroblasts. PI3K-Akt-GSK3? axis functions to promote fibroblast activation and is involved in multiple fibrotic diseases. Our data suggest that 14-3-3?+++ in PDAC tumor cells activates the PI3K-Akt-GSK-3? axis in fibroblasts which reciprocally confers PDAC tumor cell resistance to EGFR/ErbB2 TKIs. We then tested if targeting PI3K/Akt could prevent fibroblast activation and, thereby, sensitize PDAC tumor cells to EGFR/ErbB2 TKIs. Although targeting PI3K/Akt by an inhibitor (BKM120) had no significant effect on PDAC cell growth on its own, co-targeting PI3K/Akt function in fibroblasts by BKM120 and EGFR/ErbB2 activation in PDAC cells by lapatinib effectively inhibited PDAC cell growth in co-culture with fibroblasts. Therefore, we hypothesize that the vicious cycle created by the PDAC tumor cells and their fibrotic TME promotes PDAC resistance to EGFR/ErbB2 TKIs; whereas co-targeting EGFR/ErbB2 activation in the PDAC tumor cells and PI3K-Akt in fibrotic TME confers PDAC sensitivity to EGFR/ErbB2 targeted therapies. The major goals of this proposal are 1) to systematically test the therapeutic efficacy of co-targeting EGFR/ErbB2 activation in PDAC cells and PI3K-Akt signaling in fibrotic TME in various PDAC models in vitro and in vivo; and 2) to identify fibroblast-derived molecules that promote PDAC tumor cell resistance to EGFR/ErbB2 targeted therapies. The successful completion of these studies will guide the design of effective targeted therapies that can then be translated into the clinic to treat PDAC patients.